1. Field of the Invention
The present invention relates to a sheet processing apparatus which processes sheets, and an image forming apparatus having the sheet processing apparatus.
2. Description of the Related Art
Conventionally, a sheet processing apparatus is known which is equipped with a post-processing section adapted to apply post-processing such as stapling to a sheet and the sheet processing apparatus is generally connected to a latter stage of an image forming apparatus. During the post-processing, succeeding sheets cannot be conveyed to the post-processing section, which results in reduced productivity. To avoid this reduction in productivity, a technique is proposed for temporarily holding succeeding sheets on an upstream side of the post-processing section in a convey direction during the post-processing and overlaying a plurality of sheets one on top of another.
According to U.S. Pat. No. 8,511,665, the productivity is maintained as follows: during post-processing of a preceding sheet, the first one of the succeeding sheets is reversed and made to wait in a convey path, and a overlaying process is performed to overlay a sheet conveyed next on the first sheet.
In recent years, types of sheet have diversified and sheet processing apparatuses are expected to accommodate sheets ranging from thick to thin ones. Thin sheets have low rigidity, and so to say, poor strength, and thus when stacked on a stacking tray of a sheet processing apparatus, the thin sheets show reduced stackability. It should be noted that the stackability is a degree of dispersion in stack position of a plurality of sheets in the convey direction of the sheets and in a direction orthogonal to the convey direction, respectively, where the lower the degree of dispersion, the higher the stackability. Thus, a technique is generally known of performing the overlaying process described above and ejecting a plurality of sheets in a overlaid state onto a stacking tray in the case of sheets having poor strength with possibly reduced stackability even when the post-processing is not performed.
The sheet processing apparatus according to U.S. Pat. No. 8,511,665 has an upper ejection slot and a lower ejection slot used to eject sheets received from an image forming apparatus, as well as two convey paths used to convey sheets to the respective ejection slots. One of the convey paths (lower convey path) is led to the post-processing section and further connected to the lower ejection slot on a downstream side of the post-processing section in the convey direction. The other of the convey paths (upper convey path) is connected to the upper ejection slot without through the post-processing section. A convey route is switched to either of the two convey paths depending on the position of a flapper. The overlaying process is carried out using the lower convey path.
However, as described above, to improve the stackability, it is desirable to carry out the overlaying process in the case of the thin sheets even when post-processing is not performed. A plurality of sheets having undergone the overlaying process need to be conveyed to an appropriate output slot depending on whether or not the post-processing is necessary. That is, the sheets needing the post-processing should be conveyed to the lower ejection slot while the sheets not needing the post-processing should be conveyed to the upper ejection slot.
The sheet processing apparatus according to U.S. Pat. No. 8,511,665 performs the overlaying process through switch-back convey using the lower convey path. That is, the sheet conveyed in a reverse direction after once entering the lower convey path is received in a buffer section and then conveyed with a succeeding sheet overlaid thereon. Such a configuration of overlaying the sheets through the switch-back convey needs a flapper switching action in order to switch the convey route, thereby conveying the stacked sheets to the upper ejection slot. For example, if the flapper is initially positioned on a side of the lower ejection slot, it is necessary to drive the flapper to switch the convey route to the upper ejection slot.
Consequently, the flapper switching action occurs whenever the sheet is ejected, and generates noise due to the flapper hammering the convey the path. In particular, when the overlaying process of overlaying a plurality of sheets is repeated during execution of a job, the flapper switching action occurs whenever the overlaying process is performed, thereby increasing the frequency of noise generation.
In these days when noise reduction is expected of apparatus, the frequency of occurrence of the flapper switching noise is desired to be low. However, in particular, small-size sheets generally requires less convey time intervals; accordingly, the convey route switching actions take place frequently, thereby providing a problem of increasing the frequency of occurrence of flapper switching noise.
On the other hand, even if it is necessary to reduce the frequency of occurrence of the flapper switching noise, there has to be avoided a situation in which a sheet will temporarily jump out of an ejection slot in a stage of the overlaying process, which needs to be noted especially in the overlaying process of large-size sheets.